Control field event detection in a digital video recorder

ABSTRACT

The invention concerns a method and system for identifying commercial message segments of a video signal. A set of rules is applied for determining the occurrence of commercial messages with a higher degree of accuracy as compared to conventional methods. In general, method includes the steps of monitoring a digital bit stream comprising the video signal, detecting a change in a control field of the digital bit stream, and then selectively generating a commercial event notification responsive to the detecting step. According to one aspect of the invention, a change in at least one of a video sequence header and a sequence display extension can be detected. Alternatively, or in addition to checking for a change in control field, the method can comprise the step of detecting a change in an informational parameter of the video signal exclusive of the audio-visual content.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The invention concerns digital video recorder, and moreparticularly a method and apparatus for removing commercialinterruptions from video programming.

[0003] 2. Description of the Related Art

[0004] Various devices have been developed to enable consumers to recordvideo and/or audio programs for later presentation. Such devices includetape recorders, videocassette recorders, recordable compact discs, andmost recently, recordable digital video discs (DVD). Hard drives andmagneto optical discs have also been used.

[0005] One feature that is desirable for an MPEG media recorder is theability to automatically identify and selectively skip segments of arecorded video signal. For example, such a feature may be useful forautomatically editing out commercial messages from recorded televisionbroadcast signals. Commercial skip is an important feature in the fieldof videocassette recorders. In this regard, various systems have beendisclosed in the context of video cassette recorders to address thisproblem.

[0006] Conventional commercial skip technology in VCRs has used fadingto dark frames as a cue for identifying and deleting commercialadvertisements. These dark or black frames are used to generate a map ofpossible commercials in recorded television programming. However, thisconventional approach is not entirely satisfactory because it requiresone to wait while the system returns to the beginning of the segment tomark the commercial, and it uses tape sync pulse encoding ofevents/Startskip/StopSkip. Moreover, because conventional VCRs do notuse MPEG type encoding, they cannot take advantage of more sophisticatedvideo processing methods that can be applied for detecting video programtransitions such as may occur during commercial advertisements.

[0007] In recent years, more advanced systems that record video programsto a hard disk have also implemented commercial skip features. Suchsystems use a specific set of rules for identifying commercials andcommercial groups. In general, commercials are separated from each otherand from programs by video fades to black, and audio fades to silence.Accordingly, commercials can be identified by the occurrence of certainevents that are simultaneous black video and no audio. A commercialskipping device can be configured to determine when a video signal issufficiently black and the audio is at a sufficiently low level so as todetermine the occurrence of an event.

[0008] Commercial groups are groups of individual commercials that areto be skipped. The various rules for determining commercial groups arerules specifying commercial lengths, and number of commercials together.For example, if there were five events detected, with exactly one minutebetween them, it can fairly be assumed that these collectivelyrepresented a commercial group. The goal is to skip commercial groups.Thus, for example, the device can use “A” marks at the beginning of thecommercial group (to tell the device to start skipping), and “B” marksat the end of the commercial group (to tell the device to stop skippingand resume normal video and audio output). However, these systemscontinue to use techniques based on principals of analog tape recording,and the skip information is a simple manipulation of the control trackpulse width.

[0009] SUMMARY OF THE INVENTION

[0010] The invention concerns a method and system for identifyingcommercial message segments of a video signal. A set of rules is appliedfor determining the occurrence of commercial messages with a higherdegree of accuracy as compared to conventional methods. In general, themethod includes the steps of monitoring a digital bit stream comprisingthe video signal, detecting a change in a control field of the digitalbit stream, and then selectively generating a commercial eventnotification responsive to the detecting step. According to one aspectof the invention, a change in at least one of a video sequence headerand a sequence display extension can be detected.

[0011] Alternatively, or in addition to checking for a change in controlfield, the method can comprise the step of detecting a change in aninformational parameter of the video signal exclusive of theaudio-visual content. For example, this can include detecting a changein copyright information encoded in the digital bit stream or a changein GOP (“group of picture”) composition information. The GOP compositioninformation can be a number of frames or a picture type sequenceinformation for each GOP. The picture type sequence information can bebased on an order of I, B and P pictures in each GOP.

[0012] The method can also include detecting the presence of a splicetable in the digital bit stream. The data contained in the splice tablecan include timing information pertaining to the occurrence ofcommercial messages. This additional data can be used to help verify thepresence of a commercial message.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a block diagram of a digital video device that is usefulfor describing the inventive arrangements.

[0014]FIG. 2 is a flow chart illustrating a process that can be used forrecording digital data identifying a location of a commercial message ina video presentation.

[0015]FIG. 3 is a flow chart illustrating a process that can be used forskipping a commercial message in a video presentation.

[0016]FIG. 4 is a flow chart that illustrates an exemplary process bywhich one or more of rules can be combined to produce more accuratecommercial message detection.

DETAILED DESCRIPTION

[0017]FIG. 1 is a block diagram of a digital video recording system 100that can be used in accordance with the inventive arrangements. A systemcontroller such as microprocessor 120 can be used for controlling theoperation of the system and performing selected signal processing tasks.Microprocessor 120 can preferably include a suitable high-speed memory(not shown) as is convention for such devices. Control inputs formicroprocessor 120 can be received from user interface 130 via controlbuffer 128. Microprocessor 120 can communicate with the various systemcomponents as shall hereinafter be described by means of system bus 126.In FIG. 1, connections to the system bus 126 from each of the variouscomponents have been omitted from the drawing for greater clarity.

[0018] The system 100 can comprise an RF tuner section 102 for receivingand down-converting an RF video signal. The RF tuner section can converta selected channel of analog or digital RF video to a baseband signalthat can be fed to an analog output section 104 or a digital outputsection 106.

[0019] Conventional analog video and audio circuits 108 are preferablyprovided for processing baseband analog video signals received from theRF tuner analog output section 104. The output of analog video and audiocircuits 108 can be communicated directly to a video display monitor fordirectly viewing a video presentation. In addition, a digital encoder116 is preferably provided for converting the analog video and audiooutput signal into a digital format. According to a preferredembodiment, the digital format can be an MPEG format or any variantthereof such as MPEG-1 or MPEG-2. However, it should be understood thatthe invention is not limited in this regard and any suitable digitalvideo format can be used.

[0020] Once converted into a digital format, the video presentation canbe stored on a storage medium 124. Storage medium 124 can be any one ofa variety of data systems that are capable of storing digital video datafor later presentation. Such devices can include, but are not limitedto, recordable compact discs, recordable digital video discs in variousformats including DVD−R, DVD+RW, DVD−RW, DVD−RAM. Magnetic hard drivesand magneto optical discs can also be used for this purpose.

[0021] In the case where the tuner 102 receives RF video signals in adigital format, these signals can be processed in digital section 106. Abaseband digital video signal can then be passed from digital section106 to digital decoder 109. Decoder 109 is preferably configured forconverting the received digital format baseband video signal andconverting it to an analog output signal for display. For example, thedigital decoder 109 can be an MPEG digital decoder. In that case it ispreferably configured for receiving an MPEG video data signal and usingthat data to recreate audio and video information that was compressedusing systems such as MPEG-1 and MPEG-2. The digital decoder 109 caninclude digital circuits 110 for parsing the digital bit stream, andproviding all the data structures and digital information for thedigital event detector. The digital circuits 110 also provide digitaldata to drive D/A (digital to analog) converters that are preferablyincluded in the Analog Output block 112.

[0022] The digital circuits section 110 can communicate the decodeddigital video output signal to analog output section 112 where a digitalto analog converter (not shown) and other necessary circuitry can beprovided for converting the signal to a conventional analog videoformat. Subsequently, the analog video signal from analog output 112 canbe communicated to the display unit output for display.

[0023] One or more event detectors 114, 118, 122 can be provided fordetecting the occurrence of a commercial message event. Event detectors114 and 118 are preferably digital event detectors configured fordetecting events in a digital video signal using the rules describedbelow. By comparison, event detector 122 can detect events in an analogvideo signal.

[0024] Commercial message events are any combination of video signal orprogram characteristics that can be used to identify the occurrence of acommercial message. The combination of one or more of these “events” canbe used to identify the occurrence of a commercial message. Eventmessages can be communicated from the event detectors 114, 118, 122 tomicroprocessor 120 through system bus 126.

[0025] It is difficult to ensure 100% accuracy when implementing acommercial detection and skip feature. However, one or more rules can beused to increase the accuracy of detecting events corresponding to theoccurrence of commercial messages. In the present invention, the digitalevent detector 114, digital circuits 110 and microprocessor 120 can worktogether to detect events. According to a preferred embodiment, thedigital event detector 114 can check the data structures in the digitalcircuits and look for changes in the incoming video to determine when anevent has occurred.

[0026] Event Detection Rules

[0027] According to a preferred embodiment, one or more rules can beused to determine when an event has occurred, by checking for variousparameters that have changed, or gone to some particular state. Theinvention uses intelligence (microprocessor and digital event detectorlogic) to implement one or more of these rules to identify commercialsand commercial groups. All of these events may not occur for eachcommercial, but by checking for the occurrence of one or more of theseevents the invention increases the probability of accurately detectingcommercials and/or groups of commercials.

[0028] Conventional commercial detection systems have generally reliedupon changes in audio-video content to identify the occurrence ofcommercial messages. For example, detection of fades to black coupledwith silence or low audio have been used for identifying the occurrenceof commercial messages. The present invention represents a departurefrom these prior art techniques by implementing rules that are based onchanges in one or more of a digital video control field, splice pointdata tables and/or an information parameter of the video signalexclusive of the audio-visual content. Information parameters can bebased on any information directly or indirectly contained in the digitalvideo bit stream, exclusive of the audio-visual content of the video.Examples of information parameters can include coded copyrightinformation and derived information. The derived information can includethe structure and composition of groups of pictures (GOP's). Controlfields are digital data fields that help control the playback of digitalvideo, but do not generally include audio-visual content. Examples ofcontrol fields include video sequence and video sequence headers,sequence display extensions, and any other data fields that do notcontain audio-visual content that is normally displayed. A more detailedexplanation setting forth the manner in which this information can beused to detect commercial messages is set forth below.

[0029] Finally, the rules can make use of additional data that may beavailable to help verify the presence of one or more commercialmessages. For example, splice point data that may be encoded in thedigital video data can be used for this purpose. Similarly, conventionalcommercial detection methods that rely upon black frames and audiosilence can also be used for more accurate results.

[0030] Copyright Extension

[0031] According to one aspect of the invention, the event detector canmake use of copyright information encoded in a digital bit stream tohelp detect the presence of a commercial message. The program materialmay have copyright information, and when a commercial is transmitted itmay have other copyright information or maybe none at all. Detecting achange would certainly help identify commercials.

[0032] For example, in the case of MPEG-2, the event detector canutilize information provided by digital circuits 110 to check forchanges in the “Copyright Extension” information as defined in Section6.2.3.6 of ISO/IEC 13818-2:2000 “Information Technology-Generic Codingof Moving Pictures and Associated Audio Information: Video” (hereinafterreferred to as “ISO/IEC-13818-2”). This document sets forth aninternational standard for MPEG picture coding and is available from theInternational Organization for Standardization.

[0033] According to a preferred embodiment, the “video data stream” canbe parsed using conventional techniques in the digital circuits 110.When the copyright extension is encountered, the copyright extensionstructure data is made available to the digital event detector 114. Inthe case of MPEG-2, the copyright information is defined in section6.3.15 of ISO/IEC-13818-2. Section 6.3.15 defines the followingparameters:

[0034] Extension_start_code_identifier-4 bits. Identifies start ofcopyright structure.

[0035] Copyright_identifier—8 bits. Identifies copyright authority.

[0036] Original_or_copy—1 bit. 1=original, 0=copy.

[0037] Copyright_number—total of 64 bits. Actual copyright number.

[0038] One or more of these parameters can be monitored for change. Forexample, the copyright number can be monitored for change. Programmaterial will typically have a different number as compared tocommercials. Changes indicate an event. The Original_or _copy bit can bemonitored for change, as this would indicate an event. TheCopyright_identifier can be monitored for change, as this would indicatean event. Additionally, the “Copyright Extension” may exist only forprogram material and not commercials, and vice-versa. Therefore, thisstructure may come and go, at these transitions. Therefore, monitoringthe occurrence or non-occurrence of the Extension_start_code_identifierwould help indicate an event. In the case of theExtension_start_code_identifier, the occurrence or non-occurrence maynot be conclusive as to the presence of a commercial message since thisinformation might not be continuously sent)

[0039] Video Sequence Information

[0040] The digital circuits 110 can also parse the “video data stream”.When a “video sequence and video sequence header” are encountered thistemporal occurrence can also be made available to the digital eventdetector 114. The structure of this information is defined inISO/IEC-13818-2, section 6.2.2 and 6.2.2.1. Section 6.1.1 of thestandard states that the video sequence commences with a “sequenceheader”. These then indicate beginnings (and endings) of video. Theseare sent relatively frequently, for example, every 2 seconds. Ifcommercials were digitally spliced into a stream, it would probablybegin with a new “video sequence”. Therefore, the timing of these couldbe used to indicate more accurately when an event occurred.

[0041] Sequence Display Extension

[0042] The digital circuits can also parse the “video data stream” forthe occurrence of the “sequence display extension”. When the “sequencedisplay extension” is encountered, its structure data can also be madeavailable to the digital event detector 114. The sequence displayinformation is defined in ISO/IEC 13818-2, section 6.2.2.4 and section6.3.6. The document defines several parts of this structure as:

[0043] Extension_start_code_identifier—4 bits. Identifies start of thisstructure.

[0044] Colour_description—1 bit. 1=colour_primaries andtransfer_characteristics structure information does exist, andimmediately follows. colour_primaries—8 bits. Defines color relatedinformation intended for the display. For example this accuratelydefines the colors red, green, and blue, and white. There are 5 specificcolour primaries defined.

[0045] Transfer_characteristics—8 bits. Defines brightness relatedinformation intended for the display. For example this accuratelydefines what the display should do when the signal data increases invalue. That is, should the display increase in brightnessproportionally, or increase exponentially. There are 7 specific transfercharacteristics defined.

[0046] According to a preferred embodiment of the invention, thecolour_primaries data and transfer_characteristics data can be monitoredfor change. Program material may have different colour_primaries dataand transfer_characteristics as compared to commercials. Changesindicate an event. The Colour_description bit is monitored for change,as this would indicate an event. Additionally, the “sequence displayextension” structure itself may exist only for program material and notcommercials, and vice-versa. Consequently, this structure may come andgo at these transitions. Accordingly, monitoring thesequence_display_extension_ID, for occurring and not occurring, wouldindicate an event.

[0047] Group of Pictures Information

[0048] The event detector can also look at the MPEG “group of picture”(GOP), which is composed of I-frames and optionally B-frames andP-frames. I-frames start every new group of pictures, and typically arefollowed by B-frames and P-frames. These frames are compressed picturesor parts of the picture to ultimately be displayed.

[0049] Section 6.2.3 of ISO/IEC 13818-2 defines these frame types as“picture_coding_type” in the “picture header” structure as follows:

[0050] Picture_coding_type—three bits.

[0051] From ISO/IEC 13818-2 table 6-12 the types are: Bits Coding Method001 intra-coded (I) 010 predictive-coded (P) 011bidirectionally-predictive-coded (B)

[0052] The number of the ‘frames’, in each GOP and order of the I, B, Pframes is determined by the MPEG encoder used, and the encodingoperator. For example, the amount of compression selected by theoperator, or automatically implemented by the encoder may affect thelength of the GOP and the order of the frames. Therefore, the number offrames in the GOP and the order of the I, B, P frames may differ fromprogram material and commercial advertisements. The digital eventdetector 114 would preferably monitor these parameters and look forchanges, which could indicate an event.

[0053] Splice Points

[0054] SMPTE Standard “312M-1999 Splice Points for MPEG-2 TransportStreams”, defines table constructs, that are to be used for networks tosignal downstream (local) stations as to when to insert or splicecommercials into the program stream. As defined by the SMPTE Standard“312M-1999”, these tables pertain to a specific program and are carriedin program ID (PID) stream referred to by the specific program's programmap table. These tables define a schedule for splice points, that is,beginning and end times, for the insertion of commercials. The tablesalso define when to execute the insertion, as well as “pre-roll”downstream warning flag. These are called splice_command_types and are 8bits, defined as follows: splice_command_type value Command 0x01 Preroll0x02 Execute 0x03 Schedule

[0055] The digital decoder 109, provides transport stream informationand structures to the digital outputs 110. Accordingly, the eventdetector 114 can check for the presence of these tables. Information mayor may not exist in these tables, as they may be empty, or downstreamstations may have stripped the data out, or the data could beinaccurate, or the data could be accurate. If the data does not exist inthese tables, then they cannot be used. If data does exist in thesetables, then they would be useful in checking the invention's estimateof when commercials have occurred. If the times correspond, then thereis greater assurance that the commercial time, determined by theinvention, is correct.

[0056] Dark Frames and Low Audio

[0057] Currently, in the United States, most programs fade to black, andthe audio is faded to silence (or close to black and silence),separating commercials and programs. Individual commercials are alsoseparated by these events.

[0058] In the digital domain, the event detector 114 can check fordetection of video going to “dark” (black), and audio going low. Thiscan be accomplished in a variety of ways. According to a preferredembodiment, the video can be decoded and stored in a frame bufferincluded as part of the digital circuits 110. The digital event detector114 can scan each frame, checking to determine if all the digital valuesare black, representing a black frame. In actual practice there will bea predetermined, “low level” value, where any higher values, are nolonger considered “black”. In the digital domain, the event detector 114can also check for low audio. For example, the audio can be decoded andkept in an audio buffer, in the digital circuits 110. The digital eventdetector 114 can then scan this buffer, checking that all the digitalvalues are ‘silence’, representing silenced audio. In actual practicethere will actually be a predetermined, “low level” value, where anyhigher values, are no longer considered ‘silence’. This technique hasthe advantage of being available for use with analog TV programs that donot include the digital data required by the other rules as describedherein for identifying the occurrence of events. Using digital eventdetector 118 and the associated video and audio buffers described above.

[0059] Event detectors 114, 118, 122 can be programmed or pre-configuredwith one or more of the foregoing set of rules for determining theoccurrence of an event. Commercial groups comprising two or morecommercial messages can be identified using similar techniques. Forexample, it is conventional to make use of rules that will identify theoccurrence of a commercial group when a number of events are detected byan event detector 114, 118, 122 with standard commercial lengths betweeneach event. Examples of standard commercial lengths include but are notlimited to 1 minute, 30 seconds and 15 seconds. Other methods foridentifying the occurrence of a commercial message or messages in avideo signal are also known in the art and the invention is not intendedto be limited to the exclusive use of the foregoing rules for indicatingthe occurrence of an event. Instead, event detectors 114, 118, 122 canbe any combination of software and/or hardware that will generate asuitable output signal or notification when an event is detected.

[0060]FIG. 4 is a flow chart that illustrates an exemplary process bywhich one or more of the foregoing rules can be combined to produce moreaccurate commercial message detection. The process can begin in step 402by monitoring a digital video bit stream. In steps 404, 408, and 416,the system can check respectively for a control field change, aninformation parameter change, and for frames that are dark and have lowaudio. In each case, if such a change is detected it can be evaluated instep 406 to determine if it is of a nature and quality so as to indicatethe possible occurrence of a commercial message. If so, then an eventnotification can be generated in step 410. In step 410, an evaluation isconducted to determine whether the event notification generated in step410 indicates the occurrence of a commercial message or group ofmessages. In making this determination, the system can rely upon one ormore event notifications that may be stored by the system in step 414.If the system determines that the event notifications indicate thepresence of a commercial message with a pre-determined level ofcertainty, then the system can mark the commercial message in step 412.This marking can be accomplished by modifying the digital bit stream asit is recorded and/or by storing the commercial message location/timingdata in a separate file containing this information.

[0061] Splice point data in step 418 can also be used to generate eventnotifications. The splice point data can be included as part of theevent notification generated in step 410. This data can then be used tohelp verify the presence of a commercial message in step 410.

[0062] It will be appreciated by those skilled in the art that thefunctionality provided by event detectors 114, 118 can be provided in avariety of ways. For example, these detectors can be implemented asfield programmable gate arrays (FPGA's), application specific integratedcircuits (ASIC's) or general purpose microprocesors programmed toperform these tasks. Further, the event detector functionality can beimplemented in software as part of the processing handled bymicroprocessor 120. In that case, picture data can be provided directlyto microprocessor 120 from digital circuits 110 and digital encoder 116using system bus 126.

[0063] A plurality of signal switches SW1, SW2, SW3, SW4 can becontrolled by microprocessor 120 responsive to user input or theswitches can be under automatic control in response to certain formatsof detected digital or analog video. The operation of the device in FIG.1 shall now be described relative to several operating modes withreference to the flow charts in FIGS. 2 and 3.

ANALOG NTSC OPERATION

[0064] Recording Phase

[0065] In step 202 of FIG. 2, the system can monitor control buffer 128for user inputs. In step 204, if a record command is received, thesystem can begin recording as shown in step 206. According to apreferred embodiment, the microprocessor 120 can control the device 100to set the switches in the position, SW1=A, SW2=X (any connection)SW3=A, SW4=A. Subsequently, the system can have analog signals fromanalog tuner output 104 go to the analog video and audio circuits 108.In step 208, digital event detector 118 can monitor the video signalfrom encoder 116 to detect the occurrence of commercial events. Sincethe coded output of the digital event detector is based on a purelyanalog signal, the event detector will generally be limited toidentifying the occurrence of events based on fades to black and lowaudio associated with the occurrence of commercial messages. If suchevents are detected, then in step 210 the event detector 118 cancommunicate this information in step 214 to the control processor, inthis case microprocessor 120. In step 218 the microprocessor 120 cantrack the events and determine when one or more commercial messages haveoccurred and where their beginning and end is.

[0066] When one or more commercial messages have been identified by themicroprocessor 120 in step 218, this information can be recorded in step220. This information can be recorded in memory associated withmicroprocessor 120, stored in a separate file on the storage medium 124,or stored as part of the MPEG digital data. In step 216, the system cancheck to determine if the recording process is complete. If not, thesystem continues the recording process.

[0067] Playback Phase

[0068] A flowchart illustrating playback operation is show in FIG. 3. Instep 302, the microprocessor 120 can monitor control buffer 128 todetermine if a playback command has been receive. If so, then in step304 the system begins playback as show in step 306. When playbackoperation begins, the switches can be set in the positions SW1=X (anyposition), SW2=B, SW3=B, SW4=X (any connection). Recorded encoded video,such as MPEG video, can be played from the recording medium 124 into thedigital decoder 109. Digital decoder 109 generates a decoded digital bitstream that is output from digital circuits 110. The digital bit streamcan be converted to an analog format in analog output 112. The analogoutput signal is communicated to a display unit through analog output112. In step 308, microprocessor 120 can use the stored informationconcerning the location of commercial messages to determine whether aparticular portion of the playback presentation corresponds to acommercial message. If not, playback continues in step 310. However, ifthe portion of the playback presentation does correspond to a commercialmessage then the microprocessor 120 can control the playback in step 312so as to skip over commercial messages, preferably without interruptionto the program. Finally, in step 314, the system checks to see ifplayback is complete. If not, playback continues in step 310.

DIGITAL OPERATION

[0069] Recording Phase

[0070] The recording process for digital operation is the same asdescribed above relative to FIG. 2, with the following exceptions. Withthe switches in the position, SW1=X (any connection), SW2=A, SW3=B,SW4=B, the system can access digital R.F. modulated signals from tuner102. The digitally encoded signals from block 106 supply decoder 109 andswitch SW4B for recording by storage medium 124. These digital signalscan be decoded in digital decoder 109 and converted to analog format inanalog output 112. The digital event detector 114 can detect commercialmessage events using the rules as previously described. Commercialmessage events are communicated to microprocessor 120. Events can bestored in memory and used by microprocessor 120 to determine thelocation of commercial messages. When one or more commercial messageshave been identified by the microprocessor 120, this information can berecorded on storage medium 124. This information can be recorded inmemory associated with microprocessor 120, in a separate file on thestorage medium 124, or as part of the MPEG signal.

[0071] Playback Phase

[0072] The playback operation in digital mode is essentially the same asdescribed above relative to FIG. 3 with the following exceptions. Inplayback mode, the switches are in positions SW1=X (any connection),SW2=B, SW3=B, SW4=X (any connection). Encoded video, for example MPEGvideo, can be played from the storage medium 124 into the digitaldecoder 109. The digital decoder decodes the MPEG video and communicatesthe digital bit stream to analog output 112. The analog output signalfrom analog output 112 is sent to the display unit. Using the storedinformation concerning the location of commercial messages,microprocessor 120 can control the playback so as to skip overcommercial messages, preferably without interruption to the program.

[0073] According to a preferred embodiment, event start information andevent end information can be used to identify the location of acommercial message in a video presentation. This start and endinformation can be stored in any convenient manner. For example, each ofthe start and end location of the commercial message can be identifiedby an elapsed time since a start of recording of said video signal.Alternatively, a frame number can be used for this purpose. The framenumber can correspond to a number of frames of the video signal thathave been recorded since the start of recording. In either case itshould be understood that the invention is not limited to any specificlocation identifying indicia.

[0074] Data identifying an event start and event end can be stored in adata file separate from the recorded video presentation. Microprocessor120 can then access the data file for digital data identifying eventstart information and event end information, prior to the skipping step.

[0075] Instead of, or in addition to storing event start and stop datain a separate data file, such data can be recorded as part of a digitaldata stream comprising the recorded video presentation. For example, ifthe digital encoding format is MPEG the event information can berecorded following an MPEG picture header in the MPEG field identifiedas “extensions_and_user_data(2)”. This field is provided after eachpicture header in an MPEG encoded signal, following the“picture_coding_extension( )” field. Bits can be defined in theextensions_and _user_data(2) field to indicate the start or stop events.This field can also be used to indicate that the particular picture ispart of a commercial message In the event that start and stop event datais encoded in this manner, decoder 109 is preferably configured todetect and respond to such indications appropriately. For example, anotification can be sent to microprocessor 120 when an event has beendetected. The microprocessor can then selectively control playback toremove the display of the commercial message.

[0076] If the start event and end event data is stored as part of thevideo bit stream, the microprocessor can use this data to skip theportions of the video presentation associated with the commercialmessage. A video buffer can be provided in order to avoid any apparentinterruption of the video programming. To avoid the need for a largebuffer, the start event data encoded in the extensions_and_user_data(2)field can be modified after recording to also indicate the location ofan end event. In this manner, when a start event is encountered themicroprocessor can immediately determine the location of the end eventand direct the decoder 109 to skip to that location.

[0077] According to an alternative embodiment, a digital data streamcomprising the recorded video presentation can be modified by entirelydeleting from the digital data stream the commercial message portion ofthe video signal. Using this approach will permit the benefit ofcommercial message removal to be gained on players without the eventdetectors and associated processing circuitry as described herein. Themicroprocessor 120 can access event start information, event endinformation, and a selected portion of the digital data stream. Thedigital data stream can then be modified to remove the commercialmessage portion of the video signal.

What is claimed is:
 1. A method for identifying commercial messagesegments of a video signal, comprising the steps of: monitoring adigital bit stream comprising said video signal; detecting a change in acontrol field of said digital bit stream; and selectively generating acommercial event notification responsive to said detecting step.
 2. Themethod according to claim 1, comprising the step of detecting a changein at least one of a video sequence header and a sequence displayextension.
 3. The method according to claim 1 comprising the step ofdetecting a change in an informational parameter of said video signalexclusive of said audio-visual content.
 4. The method according to claim3, comprising the step of detecting a change in copyright informationencoded in said digital bit stream.
 5. The method according to claim 3comprising the step of detecting a change in GOP compositioninformation.
 6. The method according to claim 3 comprising the step ofdetecting a change in GOP composition information selected from thegroup consisting of a number of frames and a picture type sequenceinformation for each GOP.
 7. The method according to claim 6 comprisingthe further step of determining said picture type sequence informationbased on an order of I, B and P pictures in each said GOP.
 8. The methodaccording to claim 1 further comprising the steps of: detecting thepresence of a splice table in said digital bit stream; comparing atiming of said commercial event notification indicating the occurrenceof a commercial message, to commercial message insertion data containedin said splice schedule; and verifying the presence of a commercialmessage based on said comparing step.
 9. A system for identifyingcommercial message segments of a video signal, comprising: a digital bitstream monitor for monitoring a digital bit stream comprising said videosignal; a digital video processor for detecting a change in a controlfield of said digital bit stream; and event notification meansselectively generating a commercial event notification responsive tosaid detected change.
 10. The system according to claim 9, wherein saiddigital video processor further comprises means for detecting a changein at least one of a video sequence header and a sequence displayextension.
 11. The system according to claim 9 wherein said digitalvideo processor further comprised means for detecting a change in aninformational parameter of said video signal exclusive of saidaudio-visual content.
 12. The system according to claim 11, wherein saiddigital video processor further comprises means for detecting a changein copyright information encoded in said digital bit stream.
 13. Thesystem according to claim 11 wherein said digital video processorfurther comprised means for detecting a change in GOP compositioninformation.
 14. The system according to claim 11 wherein said digitalvideo processor further comprises means for detecting a change in GOPcomposition information selected from the group consisting of a numberof frames and a picture type sequence information for each GOP.
 15. Thesystem according to claim 14 wherein said digital video processorfurther comprises means for determining said picture type sequenceinformation based on an order of I, B and P pictures in each said GOP.16. The system according to claim 9 wherein said digital bit streammonitor further comprises means for detecting the presence of a splicetable in said digital bit stream; and comparing means for comparing atiming of said commercial event notification indicating the occurrenceof a commercial message, to commercial message insertion data containedin said splice schedule and for verifying the presence of a commercialmessage based on said comparing step.